Baggerman, T. and DeBari, S., 2007, The origin of a diverse suite of late Pleistocene andesitic to dacitic lavas from the northern Cascade Arc at Mt. Baker, Washington, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract V32C-04

The origin of a diverse suite of late Pleistocene andesitic to dacitic lavas from the northern Cascade Arc at Mt. Baker, Washington, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract V32C-04

Mt. Baker, a dominantly andesitic stratovolcano, is located in the northernmost segment of the Cascade magmatic arc. The origin and evolution of andesites in the northern Cascades has thus far not been explored in any systematic way. This study highlights the geochemical diversity of andesites erupted from Mt. Baker and describes processes that are responsible for the generation of this dominant intermediate lava type. Presented here are petrographic observations, mineral chemistry, major oxide concentrations, and the largest trace and REE data set to date for three Late Pleistocene and Holocene lava flows from Mt. Baker: the basaltic andesite of Sulphur Creek (52.5-57.6 wt.% SiO2, 4.7-5.5 wt.% MgO), the andesite of Glacier Creek (59.5-63.3 wt.% SiO2, 4.7-5.0 wt.% MgO), and the andesite and dacite of Boulder Glacier (60.5-64.1 wt.% SiO2, 2.1-3.5 wt.% MgO). The data are used to characterize and asses the relationship between the three flows. The three lava flows are classified as medium K, and to a lesser degree, high K, calc-alkaline basalts through dacites. Major oxide concentrations for Sulphur Creek and Boulder Glacier lavas form curvilinear trends with increasing SiO2. The andesite of Glacier Creek has distinct major oxide chemistry, with elevated concentrations of MgO and CaO and lower concentrations of Na2O and K2O for a given wt.% SiO2 relative to the major oxide trends of the other lavas. Glacier Creek lavas also show enrichment of Ni, Cr, and Sr and depletion in La, Nb, Ta, Zr and Y. REE patterns and slopes are distinct for each flow, but are not correlated with degree of differentiation. The mafic lavas of Sulphur Creek have the highest REE abundances relative to the other lavas, with the lowest La/Yb (~4.5). The Glacier Creek andesites have the lowest REE abundances and the largest La/Yb (~6.7). The Boulder Glacier andesites and dacites have intermediate REE abundances relative to the other lavas with intermediate La/Yb (~6.4). All lavas display disequilibrium textures such as magmatic reaction and resorption textures, sieved textures and complicated chemical zoning patterns typical of magma mixing and complicated fractionation processes. Major and trace element fractionation modeling of the mafic lavas of Sulphur Creek fail to produce the higher MgO values in the more differentiated Glacier Creek lavas, and the steeper and relatively depleted REE abundances in both the Glacier Creek and Boulder Glacier lavas. The above data and petrographic observations, coupled with major and trace element fractionation modeling, suggest that the generation of these chemically distinct andesites requires multiple mantle sources that have been modified by crustal processes. These processes include magma mixing and/or complicated crystal fractionation processes.